The present invention relates generally to a portable data device efficiently utilizing its available power and method thereof, and in particular, to a portable data device efficiently utilizing its available power by adjusting its clock rate.
Credit cards, typically provided with magnetic stripes, have been around for many years. These types of credit cards, however, have a design flaw. The mechanical interface between the credit card and the reader requires periodic cleaning. The poor reliability of the readers, due to the mechanical interface, causes down time for reader maintenance. Contactless smart cards have been developed which eliminate the mechanical interface between the card and the reader.
Standards are currently being created around the contactless smart card. The most widely accepted standard is the ISO-14443, which specifies the nature and characteristics of the fields to be provided for power and bi-directional communications between a portable data device (e.g., smart card) and an interface device (e.g., a reader). The system requires the presence of both the reader and the smart card. Together, the reader and the smart card comprise a loosely coupled transformer. A sinusoidal waveform, which is also the radio frequency (xe2x80x9cRFxe2x80x9d) carrier, is injected onto the reader coil (antenna) and is used to create a magnetic field. When the smart card is placed in the field, the energy that passes through a loop antenna residing on the smart card is received by an integrated circuit (xe2x80x9cICxe2x80x9d) also residing on the smart card. The power for the smart card is extracted from the magnetic field. By changing the intensity of the magnetic field as a function of time, data can also be transferred between the smart card and the reader.
FIG. 1 illustrates a block diagram of a contactless smart card system having a reader 100 and a smart card 110. The reader 100 comprises a signal source 102 and a resonant output circuit, which comprises capacitors 104 and 106 and an inductive antenna 108. The resonant frequency of the resonant output circuit 104, 106, 108 is substantially equal to the frequency of signal source 102. The inductive antenna 108 generates an electromagnetic field when a signal is applied to it.
The smart card 110 comprises an integrated circuit 114 and an inductive loop 112. When the smart card 110 is brought into the proximity of the reader 100, the inductive antenna 108 of the reader 100 and the inductive loop 112 of the smart card 110 form a loosely coupled transformer. A coupling coefficient M 115 for the loosely coupled transformer is a function of distance and orientation of the inductive antenna 108 and the inductive loop 112. The electromagnetic field generated by the inductive antenna 108 is received by the inductive loop 112 and converted to a current. This received current can be used to power the integrated circuit 114. The electromagnetic field can also be used for data transfer between the reader 100 and the smart card 110.
The integrated circuit 114 can consist of several different components. A digital circuit 116 provides the xe2x80x9cbrainsxe2x80x9d and functionality for the smart card 110. The other components contained within the integrated circuit 114 support the functionality of the digital circuit 116.
The inductive loop 112 and a tuning capacitor 118 constitute a resonant tank. This resonant tank is tuned to the signal frequency of the signal source 102 of the reader 100. The resonant tank facilitates efficient power coupling of the received field to the integrated circuit 114.
A power rectifier 120 rectifies the alternating current (xe2x80x9cACxe2x80x9d) signal received on the inductive loop 112 creating a signal with direct current (xe2x80x9cDCxe2x80x9d) content. The power rectifier 120 essentially performs an AC-to DC transformation. A power controller 12 operates on this DC signal and creates the required power supply signals required to power the digital circuit 116.
A receiver 124 performs data detection and reconstruction. The receiver 124 detects and reconstructs the digital bit stream of any signal transmitted by the reader 100 to the smart card 110. The receiver 124 supplies input data for the digital circuit 116. A transmitter 126 creates a modulated signal for transmission via the electromagnetic field from the smart card 110 to the reader 100. The transmitter 126 provides the output data path for the digital circuit 116.
A timing reference is created by a clock generator 128, which creates a clock signal from the received signal. The clock generator 128 provides the timing reference for the digital circuit 116.
FIG. 2 illustrates how the power level at the smart card 110 changes as the distance between the reader 100 and the smart card 110 varies for the smart card system shown in FIG. 1. Curve 201 shows the power available to the smart card 110 for varying distances between the inductive antenna 108 and the inductive loop 112. As can be seen as the smart card 110 moves closer to the reader 100, the power received is greater than what is available at further distances. As the distance between the inductive antenna 108 of the reader 100 and the inductive loop 112 of the smart card 110 increases, the power available to the digital circuit 116 decreases. Due to integrated circuit requirements, the excess power received at close coupling between the smart card 110 and reader 100 should be consumed. Currently, the excess power is wasted by dumping it to ground. To allow operation of the smart card 110 over a range of distances, the power levels are set so the operating power of the smart card 110 is obtained at the desired maximum distance between the reader 100 and the smart card 110. Since there is no feedback between the smart card 110 and the reader 100, the power level cannot be adjusted during a transaction.
As the complexity of the smart card system increases, so will the power required by the smart card 110 to support the increased card capabilities. Due to emission standards, which support the smart card system of FIG. 1, the amount of power delivered by the reader to the smart card 110 cannot be increased. So as can be expected, the operating range of the smart card 110 is reduced when additional system features/requirements are added to the smart card 110. This reduction in operating range degrades system performance.
Further, it is generally accepted by the smart card industry that the transaction times must be less than 100 milliseconds. The overall transaction time is a function of the time required to transfer information between the reader 100 and the smart card 110. As can be expected, additional features increase the time required to complete a transaction. ISO specifications dictate the nature and characteristics of the carrier frequency to be provided for power between the reader 100 and the smart card 110. Since the frequency delivered to the smart card 110 is dictated by the ISO standards, and the clock rate is derived from the frequency, the number of clock cycles during a given time period is a constant. Additional commands, however, require more clock cycles, thus increasing transaction time.
Moreover, for prior art smart card designs, such as that shown in FIG. 1, there is a fixed amount of useful power dissipation. The integrated circuit 114 operates using a constant clock frequency independent of the distance between the reader 100 and the smart card 110. Since power dissipation in a digital complementary metal oxide semiconductor (xe2x80x9cCMOSxe2x80x9d) circuit is directly proportional to the clock frequency, the amount of power dissipated by the CMOS digital circuit with a fixed clock frequency will also be fixed. If this fixed power dissipation requirement is met by the received power available in the RF field, then the smart card 110 will operate as desired. If the fixed power dissipation requirement is not met by the received power in the RF field, the smart card 110 will not function.
FIG. 3 illustrates a plot of current versus distance for the smart card system shown in FIG. 1. Curve 303 shows the current available from the RF field. Curve 305 shows useful current dissipation versus distance. Note that curve 305 is a fixed constant value for distances where more power is available than is required. When the current available from the RF field, as represented by curve 303, drops below the fixed power requirement for the smart card 110, the useful power dissipation drops to zero, as the smart card 110 is no longer functional or useful. The difference between the curves 303 and 305 represents wasted excess power that provides no functional benefit. This is power, however, that should be dissipated by the smart card 110.
Thus, there exists a need for a circuit that will allow additional power consuming features to be added to a smart card, without increasing the power delivered to the smart card. Additionally, there exists a need for a circuit that will allow faster transaction times without increasing the frequency delivered to the smart card from the reader.